A person was asking me how the key of the bellowphone is determined. He noted that it seems to be happiest in D, which is correct, except it is actually a few cents shy of D: flat, you know, not the other thing. The bellowphone can also play in A to some extent, and by dint of extreme coercion and contortions, it can be made to play in F# and others. This is entirely due to the chaotic arrangement of the pipes, which were added as required for specific pieces, not from any coherent plan. As the repertoire grew, I would either add notes, or just try to work with what was there.
The kazoo has nothing to do with determining the key of the music, as my friend erroneously suggested; the kazoo is limited only by my singing ability. With my voice alone, I can theoretically sing in Harry Partch's 43 tone scale. In fact, I usually do sing that way, although not on purpose, I can assure you.
So the theory of the Bellowphone, is in fact, lack of a theory.